The present invention is applied to a so-called "direct drive motor" which drives directly a robot or an office machine without using a reduction gear.
More particularly, in a low speed high torque motor, the present invention relates to an improved field system and its production method.
A heretofore known motor which generates high torque at a low rotating speed has teeth on a rotor and a stator, and a field magnet inside the rotor so as to utilize effectively the magnetic flux of the field magnet to obtain the large torque. The content of U.S. Pat. No. 4,687,961 and Japanese Utility Model Laid-Open No. 59-185987 (1984) also relates to a motor of this kind.
In the prior art as stated above, the field magnet is disposed in a field system of the rotor, and after the field magnet is installed in the field system of the rotor, the field magnet is magnetized by a magnetizer which is disposed outside of the rotor and generates magnetomotive force to the field magnet of the rotor in order to magnetize the magnet.
In case of using the field magnet which is already magnetized after the field magnet is installed in the field system of the rotor, machine work on the rotor must be conducted in order to keep a high field cutting accuracy of the rotor which makes a very small magnetic gap with the stator.
But the structure of the prior art described above involves the following problems.
(1) When external magnetization is effected after assembly, extremely great magnetomotive force must be generated in a magnetizer because the magnet to be magnetized and a magnetizing yoke are too much spaced apart from each other, and large-scale equipment is necessary. The production steps cannot be standardized uniformly because consistent external magnetization is not possible for the multi-layered type magnet. PA0 (2) Though the use of a magnet magnetized in advance may be possible, machine work must be conducted after the insertion of the magnet because the gap between the rotor and the stator in this kind of motor is small. Therefore, removal of chips and fins is extremely difficult and working property is greatly lowered. Moreover, magnetic dust removing equipment is necessary in the work shop and it is a primitive method, manual dust removal by use of an adhesive tape has been conducted. But it is troublesome and wastes much time.